Seems, Microsoft inadvertently had made Windows 8.1 update 1 available on its servers and before it was pulled it was installed and mirrored by many. So, now it is still available to download from many sources along with instructions on how to do it. Please do it at your own risk, however. The final Build is 6.3.9600.17031.WINBLUE_GDR.140221-1952 with release date of 21/02/2014. Check the links at end of this post.
Read more at WinCentral
